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Black hole images present an annular region of enhanced brightness. In theabsence of propagation effects, this "photon ring" has universal features thatare completely governed by general relativity and independent of the details ofthe emission. Here, we show that the polarimetric image of a black hole alsodisplays universal properties. In particular, the photon ring exhibits aself-similar pattern of polarization that encodes the black hole spin. Weexplore the corresponding universal polarimetric signatures of the photon ringon long interferometric baselines, and propose a method for measuring the blackhole spin using a sparse interferometric array. These signatures could enablespin measurements of the supermassive black hole in M87, as well as precisiontests of general relativity in the strong field regime, via a future extensionof the Event Horizon Telescope to space.

Universal Polarimetric Signatures of the Black Hole Photon Ring